This invention relates generally to frequency modulated-continuous wave (FM-CW) ranging radars and more particularly to an FM-CW ranging radar that eliminates the sidelobe ambiguity in the range response signal.
The type of radar that employs continuous transmission, either modulated or unmodulated, has had wide application. Two of the earliest applications were the proximity fuze and the FM-CW altimeter. The FM-CW radar has wide application today in fuzes that are designed to detonate at a preset distance from a predetermined target. This application requires that the radar system be simple and cost-effective, be accurate, i.e., provide a signal at the pre-set distance, and be able to function in the presence of extraneous signals caused by passive countermeasures such as corner reflectors or extraneous signals caused by active countermeasures such as ECM.
The signal generated by an FM-CW ranging radar system is known as the range response. The range response of a single channel FM-CW radar is approximately a (sin x)/x shape which consists of a mainlobe and sidelobes. The presence of sidelobes in the range response of an FM-CW radar limits the dynamic range of target amplitude which the radar can handle unambiguously. Therefore, since the first sidelobes in the range response generated by a single channel FM-CW ranging radar are only approximately 13 dB below the main lobe, the unambiguous dynamic range of the system is only approximately 13 dB. To improve the unambiguous range of the system it is necessary to either reduce or eliminate the sidelobes in the range response. The prior art, described by (1) Peperone, S. J., "Side-Lobe Suppression in an FM Altimeter," Master's Thesis, University of Md., College Park, Md., May, 1966, (2) Tozzi, L. M., "Resolution in Frequency-Modulated Radars," Ph.D. Dissertation, University of Md., College Park, Md., 1972, and (3) Mattox, B. G., "Improvement of the Range Response of Short-Range FM Radars," Ph.D. Dissertation, University of Florida, 1975, uses two general approaches to reduce the amplitude of the sidelobes. One approach is to shape the transmitted waveform and the second approach is to shape the received waveform. Practically, the unambiguous dynamic range can be increased to approximately 30 dB. Unfortunately, ECM methods can overcome this ambiguity range by generating an appropriate signal of sufficient power which results in the malfunctioning or pre-detonation of the fuze, thus either negating or reducing the effectiveness of the device.
It is therefore one object of this invention to provide an FM-CW ranging radar wherein the sidelobe ambiguity is eliminated.
It is another object of this invention to provide an FM-CW ranging radar wherein the necessity of counter ECM devices is eliminated.
It is a further object of this invention to provide an FM-CW ranging radar wherein the sidelobe ambiguity is eliminated simply and cost effectively.
It is still another object of this invention to provide a method wherein the sidelobe ambiguity in FM-CW ranging radars can be eliminated.
It is still a further object of this invention to provide a method wherein the effectiveness of devices using FM-CW radar ranging systems can be improved.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.